Before this invention there were substantially two types of metering valves. A first type is made up of a valve including a cylindrical inlet chamber equipped with a piston here inserted with clearance, the piston itself being pushed toward the lubricant inlet by a coil spring inserted downstream around the rod with which the piston is provided in order to close, by the downstream end of the rod itself, the narrowing portion through which the lubricant passes into the subsequent discharge chamber. In the discharge chamber there is contained a substantially truncated-conical rubber shutter with its stem downstream, to be guided and pushed against a truncated-conical seat of the above-mentioned narrowing portion by a conical coil spring inserted in the outlet area of the chamber itself.
A second type of metering valve has been provided, in its cylindrical inlet chamber, with an elastic rubber shutter shaped like a truncated-conical mushroom sliding in the chamber itself, the downstream end of its stem is destined to close the longitudinal hole inlet of a tubular element guided at the bottom and resting against the outlet narrowing portion of the cylindrical discharge chamber; said tubular element being equipped upstream with an external flange to be fixed in the center of the discharge chamber even if it lets the lubricant pass through the flange and the chamber themselves. This same tubular element is provided at its downstream end with a couple of notches in order to allow its longitudinal hole to communicate with the surrounding discharge chamber, wherein there is inserted a coil spring which pushes upstream a metering bushing coaxially slidably coupled, on the inside with the external surface of said tubular element end, and on the outside, by means of a flange located on the opposite side of the spring, with the cylindrical surface of the discharge chamber. Upstream of the metering bushing a sliding rubber ring is placed to seal the lubricant downstream of the flange of said tubular element, between said tubular element and said cylindrical discharge chamber surface.